It is well known to utilize fluidized beds for cleaning, classifying or other processing of particulate materials. Multiple stage fluidized beds often include a perforated horizontal plate through which a fluidizing medium, such as air, upwardly flows to fluidize particulate matter supported on the plate. Particles of, for example, higher specific gravities tend to sink to the bottom of the bed while particles of lower specific gravity float to the top of the bed. The upper particles rise to an elevation above one or more downcomer tubes, fall into the tube, and are discharged onto another perforated plate supporting a lower bed in the next lower stage. The lower particles are generally removed in a continuous or batch process through lateral pipe outlets. The fluidizing medium passes serially upwardly through the lower perforated plate, the bed supported on the lower plate, the upper perforated plate and the upper bed.
Several concerns arise in operation of such systems. For example, the downcomers present a path of lower resistance whereby the fluidizing medium tends to short circuit the particles of the upper bed-stage as well as restricting the downflow of particulates from the upper stage to the lower stage. This increases the amount of fluidizing medium required to achieve a given fluidized condition in particularly the upper bed and to achieve a given mass throughput. It also requires a larger diameter bed since the cross sectional area for fluidization is reduced by the area of the downcomers. To control the throughput, such systems also rely not only on the rate of feed into the uppermost bed, but often also on some type of valving to control flow through the downcomer. Valving or seals are often utilized to alleviate the short circuiting path. Additionally, since the downcomers are localized at various regions of the bed, the particle distribution is not even, and can provide an imbalance in the type of particles being discharged from the bed. The quality of fluidization within a bed having downcomers is also affected by an imbalanced distribution of the fluidizing medium.
It has been recognized that some of these concerns can be alleviated through use of a structure such as that disclosed in U.S. Pat. No. 3,333,692 which is useful for drying and subsequent cleaning of particulate materials. The structure and operation of the device disclosed therein is substantially different from more conventional fluidized bed classifiers or cleaners because the upper stage is designed merely to dry and dedust the particulate material. No separation per se occurs there, all of the dried and dedusted particles eventually being transferred through a downcomer to a lower bed for separation in a fluidized bed of a material, such as magnetite, having a specific gravity intermediate the high density and low density portions of the feed material. Particularly in cases where a single drying and dedusting bed is desired to be associated with more than one bed of magnetite, the product from drying and dedusting is divided into plural streams. This is accomplished by an annular hooded peripheral launder divided into sections, each having a corresponding outlet pipe, by radially positioned vertical plates. The dried and dedusted particulates overflow from the bed into the adjustable height peripheral launder, and are directed through the pipes into the magnetite bed, which can be positioned below the drying and dedusting bed. While not clearly taught, the use of a peripheral overflow launder allows the elimination of the central downcomer and alleviation of attendant concerns.
With such system, the possibility for short circuiting and the desirability of sealing means and control valves remain. However, the fluidizing medium distribution and product imbalance concerns are lessened. Where, for example, the drying and dedusting bed is only partially fluidized, the particles of higher specific gravity would tend to stagnate at the lower portion of the bed, and not overflow unless the adjustable height launder is moved to a very low position whereby the bed height becomes shallow, allowing little height for the particulate segregation and thereby the separation becomes less effective.
It is desirable to provide a fluidized classification system which alleviates the above discussed and other concerns associated with prior classifying systems.